Maps and Compass

Traditional Paper Maps

Many times, after retiring to bed, I read under the covers, torch in hand so as not to disturb my brother. Sometimes it was a novel, but often it was the 1:100,000 series map of the local mountains, with which I planned improbable fishing trips. Looking at the web of creek beds, prominent hills, mountain tops and trails, which promised accessibility to an unknown territory, I built a mental picture of the terrain and then ideas began to take shape on what I hoped would become a great adventure and was probably the start of an obsession with maps.

In an era of Google Maps, GPS receivers and digital maps loaded on your smartphone that give you your global position within metres, one would think paper maps have become obsolete! Far from it – in my mind, nothing beats a paper map.

Grid/Magnetic Angle, Canberra
Grid/Magnetic Angle, Canberra

Map in hand, you feel you own the territory. You can overview, not only your immediate area of interest but what is around it for a considerable distance. This is very hard to do on a smartphone. Also, imagine being out on a first-time trek and finding that your smartphone fails, or the navigation satellites are turned off!

Navigation with map and compass

In tandem, paper maps and smartphones work well together, combining the positioning and zoom scaling of the Smartphone, and the easy viewing and safety of a paper map. But what happens when you only have a paper map?

Map and compass, inseparable partners, are a must on any serious trekking excursion equipment list. They are a navigational aid that can help you make good decisions and get out of trouble in any weather. There are countless stories of situations where people have lost their lives from misuse or the lack of a map and compass.

Serious maps will always show the direction of the magnetic north. It is shown as a Grid/Magnetic Angle or a Magnetic Declination.

The Grid/Magnetic Angle

Grid/Magnetic Angle, Canberra
Grid/Magnetic Angle, Canberra

This angle is formed between the local grid system and the magnetic direction.

As an example, Canberra in their recent special data, uses the Geocentric Datum of Australia 2020 (GDA 2020) with Map Grid of Australia (MGA 2020 zone55) projection.

The Magnetic Declination

Magnetic Declination, Canberra
Magnetic Declination, Canberra

This angle is formed between the true north and the magnetic direction.

Usually found on older maps but still used as a reference.

Knowing how these angles apply on a map allows one to place and use a compass side by side with the map. The fundamental necessity is orienting the map using the compass. The aim is to match the direction of the magnetic north of the needle in the compass with the deduced magnetic direction on the map.

Creating a compass bearing and following it while sighting objects ahead is very useful in maintaining your direction while navigating. There is no better example of where this technique is necessary than in a white-out situation, where the visibility is very limited and one has to keep a direction to reach a target. In a group, a member can be sent ahead on the bearing to maintain direction.

The compass bearing is calculated as the angle between the direction of travel and the north of the grid then subtracting the value of the Grid/Magnetic Angle or the Magnetic Declination.

Back-bearings are also useful in determining your position when you have at least two known visible distant points on the map (three are better) and in your sight. One can sight the distant object, read the bearing and add the value of the Grid/Magnetic Angle or the Magnetic Declination,

It’s important to master these techniques before departing on important treks.

There are many articles and some books written on the importance of maps. Here are some useful links; Mapshop and PBS.

The Compass and Magnetic Field

Lodestone compass

The first compasses appeared in the Han Dynasty more than 2000 years ago, but records show that they were not used for navigation until 1000 years later. They were made of lodestone, a naturally magnetised stone of iron.

In the 7th century, Chinese scholars took the next step by magnetising iron needles, and three centuries later compasses were used for navigation. Compasses made their way to Europe around the 12th century.

What drives the compass is the Earth’s Magnetic Field. The following diagram by Henrik Mouritsen illustrates how the field works on our planet. Note that is stronger as it approaches the poles.

The Earth’s magnetic and geomagnetic fields – note that the south and north poles are reversed in the earth’s core.

The magnetic field on our earth does not run horizontally along the earth’s crust. Depending on where you are, it will point towards the magnetic poles or away from them.

A compass needle made for the northern hemisphere will still point towards the poles, but won’t sit horizontally. Compass manufacturers make compasses to suit your zone by weighting the needle.

Compasses come in all shapes and sizes, oil-filled and not. Just having a compass in your backpack is a must.

Suunto Oil filled compass
Sighting Compass
Top of the range Silva Expedition 4755 Military Compas
Top of the range Silva Expedition 4/55 Military Compass

As compasses go, my favourite is the “Silva Expedition 54”

More economical than the military version, it has a unique sighting mechanism – a prism that magnifies the measurement reading to 1-degree units.

This compass is oil filled, robust, can be used as a protractor and has map-measuring scales and a magnifying lens. In summary, it’s quite an instrument!

The process of sighting is shown in the following illustrations.

Sighting with Silva 55 Expedition
Sighting with Silva 54 Expedition
Silva Expedition 54 sighting

Placing the compass between the target and you, you look directly in the small window, through the housing of the compass, to the prism.

What you see is a central hairline. You match this hairline to your target and the reading is the bearing.

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